In one conventional process for the industrial production of a carbonic acid ester, an alcohol has been reacted with phosgene. However, this process has problems, for example, that highly poisonous phosgene is used and that the reaction between an alcohol and phosgene yields highly corrosive hydrochloric acid as a by-product in a large quantity.
Therefore, a large number of processes have been proposed for the production of a carbonic acid ester without using phosgene. Among these is a commonly employed process in which an alcohol to be esterified is reacted with carbon monoxide and oxygen in the presence of a catalyst. Representative examples of the catalyst used in this process include a catalyst comprising a copper compound (JP-B-45-11129) and a catalyst comprising a combination of a palladium compound, a copper compound, and a base (JP-B-61-8816). (The term "JP-B" as used herein means an "examined Japanese patent publication".) Further, known as an improvement of the latter catalyst is one comprising a combination of a palladium compound, a weak acid salt or/and halide of copper, and a weak acid salt or/and halide of an alkali metal or alkaline earth metal (JP-A-1-287062). (The term "JP-A" as used herein means an "unexamined published Japanese patent application.)
The above-described catalysts, however, still have problems which should be overcome in order to utilize these catalysts in the industrial production of carbonic acid esters. Illustratively stated, in the case of using a copper compound as a catalyst, the copper compound, which has a low solubility, should be used in a large amount in order to obtain an effective reaction rate, since the catalytic activity of the copper compound is generally low. However, there are cases where the copper compound as a catalyst changes into copper hydroxychloride or other compounds according to the reaction conditions used and, as a result, the activity of the catalyst decreases.
In the case of the catalyst comprising a combination of a palladium compound, a copper compound, and a base, since water and oxalic acid are formed as by-products of the reaction and accumulate with the progress of the reaction, part of the catalytic components react with these by-products and separate out as an insoluble hydroxide, oxalate, or metal, resulting in a decrease in catalytic activity. It is, therefore, difficult to industrially use the catalyst continuously or repeatedly.
As apparent from the above, an industrially important theme for the conventional processes using either the catalyst comprising a copper compound or the catalyst based on the combination of a palladium compound and a copper compound is to establish a method of recovering or regenerating the catalyst.